by Jack Rosebro
A new analysis of anthropogenic emissions as well as atmospheric concentrations of carbon dioxide from 1850 to the present day indicates that the relationship known as the “airborne fraction”—the ratio of manmade CO2 emissions to the amount of those emissions that remain in the Earth’s atmosphere—has remained remarkably constant throughout the years, varying by an average of 0.7% ± 1.4% per decade. The analysis indicates that natural carbon sinks are maintaining overall resilience despite recent signs that the carbon uptakes of specific sinks are in decline.
The study, which was conducted by Wolfgang Knorr of the University of Bristol, UK, may have implications for upcoming climate negotiations, particularly with regard to deforestation and other land-use changes.
Approximately 40-45% of carbon emissions emitted every year remain in the atmosphere, with the balance absorbed as part of the Earth’s carbon cycle. Many projections of future greenhouse gas (GHG) emissions and subsequent warming trends, particularly those used by the United Nations Framework Convention on Climate Change (UNFCCC), assume a constant airborne fraction to 2100.
However, research published after the 2007 release of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), which incorporates projections utilized by the UNFCCC, has shown a decline in specific carbon sinks such as the Southern Ocean. (Earlier post.) With newer climate-carbon cycle coupled models suggesting that a weakening of carbon sinks could add as much as net ~500 ppm carbon dioxide to the Earth’s atmosphere by 2100, concerns have been raised as to whether a relatively flat airborne fraction should be assumed for the remainder of the century.
While some aspects of the Earth’s “carbon budget” (e.g. annual increases of manmade emissions of carbon dioxide, concentrations of the gas in the Earth’s atmosphere, and the resultant uptake of carbon by natural sinks) have been relatively straightforward to calculate, quantifying the precise mechanisms by which carbon is naturally stored has proved more difficult. One knowledge gap is the so-called “missing carbon sink”, a phenomenon that has been studied extensively for the past three decades.
|Estimated carbon emissions and uptake, 1850-2000, including the Earth’s as yet unidentified carbon sink. Source: Woods Hole Research Center Click to enlarge.
For example, one estimate from the Woods Hole Research Center calculates the average annual emissions of 8.5 petagrams of carbon (PgC) during the 1990s (about three-fourths from combustion of fossil fuels and the remainder from changes in land use). With the sum of the annual accumulation of carbon in the atmosphere and the annual uptake by the oceans calculated at 5.6 PgC, an additional sink of 2.9 PgC per year exists, but cannot be accounted for.
A systematic decline in the Earth’s ability to absorb steadily increasing carbon emissions would have the net effect of forcing the increase of atmospheric greenhouse gas concentrations at a rate sharply higher than models currently project. Although most climate models do not calculate the effects of “positive feedbacks”—secondary effects of increased GHG emissions which, in turn, intensify the severity and rate of warming—efforts are underway to develop more complex models that incorporate feedback effects. A lack of accounting for feedback effects is considered to be a potential weakness of many current climate models.
However, Knorr focused on observational data, analyzing atmospheric concentrations of carbon dioxide recorded at Mauna Loa, Hawaii and the South Pole. Ice core data from Law Dome, East Antarctica was used to determine concentrations of carbon dioxide prior to instrumental records. Data on worldwide CO2 emissions from fossil fuel combustion, cement manufacturing, and gas flaring, as well as carbon flux to the atmosphere resulting from land-use changes, were provided by Oak Ridge National Laboratories.
Another concern has been the influence of land use changes on carbon uptake in tropical forests, which are generally not monitored as well as northern forests. In the absence of observational data, researchers have had to rely more heavily on climate-coupled computer models of land-use changes, with significant margins of error. Knorr found that “the statistical model of a constant airborne fraction agrees best with the available data if emissions from land use change are scaled down to 82% or less of their original estimates.”
Knorr’s data does “not necessarily” run counter to theories of weakening carbon sinks, cautions the author. “Like all studies of this kind”, he notes, “there are uncertainties in the data, so rather than relying on nature to provide a free service, soaking up our waste carbon, we need to ascertain why the proportion being absorbed has not changed.”
Knorr, W. (2009), Is the airborne fraction of anthropogenic CO2 emissions increasing?, Geophys. Res. Lett., 36, L21710, doi: 10.1029/2009GL040613